(a) Field of the Invention
The invention relates to the diagnosis of severe diseases based on the determination of the presence of AGE-IgG autoantibodies in patients.
(b) Description of Prior Art
Rheumatoid arthritis (RA) and diabetes (both type I and type II) are chronic diseases which are very costly to society, both in terms of reduced productivity and quality of life of the afflicted individuals, and in terms of the increased health care costs. The exact causes of RA and diabetes are unknown at present but appear to be multifactorial with both genetics (polygenic) and environmental factors playing important roles. We have recently discovered a new autoantigen/autoantibody system in RA which may provide insight into the pathogenesis and lead to new more effective therapies.
In patients with elevated blood glucose levels and/or oxidative stress, a large number of proteins can be non-enzymatically glycated, whereby the glucose molecules bind to the exposed amino acids lysine and arginine. Not all lysines and arginines are glycated on proteins of individuals with high blood glucose and/or oxidative stress. There are three key factors that influence the number and extent of protein glycation, namely the blood glucose level, the half-life of the protein in question (long lived proteins are more heavily glycated) and the number of lysines and arginines in the correct microenvironment in the protein. These are likely the surface exposed lysines, or as in the case of hemaglobin the amino terminal valine, which is adjacent to another basic amino acid (lysine or histadine). For example, in albumin, only 10 of 59 lysines are glycated. With time the sugar is bound irreversibly. Proteins with this modification are said to have advanced glycation end products (AGE). A well studied protein, hemoglobin-AGE (HbAlC) is used to monitor type 1 diabetics for insulin compliance. To date AGE-modified proteins have been suggested to have a role in atherosclerosis, amyloidosis and the complications of diabetes.
Antibody molecules, of all subclasses have lysine and arginine residues on their external surfaces which can be modified by advanced glycation end-products. Previous studies have shown that both IgG and IgM can be glycated (Dolhofer-Bliesener, et al., Scand. J. Clin. Lab. Invest. 50:739-746, 1990; Hennessey, et al. J. Parent. & Enter. Nutrit. 15, 60-64, 1991). The half-lives of the classes and subclasses of immunoglobulin in circulation vary, with IgGl, 2 and 4 having the longest half lives of 23 days. Previous reports have determined that there is a loss of Fc receptor function, namely in the ability to bind complement and protein A when the AGE modifications are present. There is also a report of loss of antigen binding when a mouse monoclonal was AGE modified (Sasaki et al., Clin. Chim. Acta, 220:119-121, 1993).
Up to 70% of patients with RA have circulating antibodies, generally in high amounts, called rheumatoid factors (RF) which bind to the Fc part of IgG antibodies. The presence of these antibodies is used to help diagnose the disease. Although RFs can be of any subclass, IgM RFs are the best studied. Characterization of RF structure (genes, degree of somatic mutation), binding specificity and role in disease pathogenesis has been the subject of much research. The binding site for RF binding to the IgG is in the CH2 to CH3 cleft, and based on the recent crystallographic data of a monoclonal RF completed to IgG Fc (Corper et al.. Nature Struct. Biol. 4:374-381, 1997), it appears that lysine at 437 and arginine at 255 on the IgG are in, or adjacent to the mapped binding site.
It would be highly desirable to be able to determine whether RFs could bind to AGE modified IgG.
It would be highly desirable to be provided with a diagnostic tool for the diagnosis of severe diseases based on the determination of the presence of AGE-IgG autoantibodies in patients.